1. Field of the Invention
The present invention relates to a method for improving durability of an exhaust pipe, and relates to an exhaust gas purification apparatus.
2. Description of Related Art
As a catalytic purification system for removing nitrogen oxides (NOx) in exhaust gas of an engine, an exhaust gas purification apparatus disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2005-226528 has been proposed. Such an exhaust gas purification apparatus supplies by injection a urea aqueous solution in accordance with an engine operation state on the exhaust gas upstream side of an SCR (selective catalytic reduction) converter disposed in an exhaust pipe, to thereby cause a selective reduction reaction between NOx and ammonia generated from the urea aqueous solution in the SCR converter, to purify the NOx into harmless components.
At least a part of an exhaust pipe of an engine may be made of a plate of ferritic stainless steel, such as SUS 436L, which is excellent in workability and weldability. In this case, the exhaust pipe is made up by shaping the ferritic stainless steel plate into a pipe form, butting its ends, and then welding the ends to each other.
The urea aqueous solution supplied by injection to the exhaust pipe of the engine is hydrolyzed using exhaust heat and vapor in the exhaust gas, thereby generating ammonia, hydrogen, and the like that have corrosiveness onto metal. A tensile stress may remain in the welded portion of the exhaust pipe due to the welding, so that stress cracking may easily occur therein, and further the diameter of metal crystal grains in the surface is increased by heat from the welding, so that the welded portion may become sensitive to the corrosive components.
For this reason, the durability of the welded portion of the exhaust pipe may be unfavorably reduced due to the exposure to an environment in which stress cracking may easily occur and due to a synergistic effect in combination of the tensile residual stress and the increase in sensitivity to corrosive components.